This project is concerned with the mechanism(s) of cooperation between cells in the immune response and the role of antigen, antibody and antigen-antibody complexes. The aims of the project for the current budget period are 1) to identify and characterize the suppressor function provided by the thymus during neonatal tolerance induction: this will be done using in vivo and in vitro systems for studying cell interactions in rats tolerant of BSA and methylated BSA or responding to various T and B cell mitogens. 2) to determine how the immune response of mice to DNP-bacitracin is genetically controlled, whether the anti-DNP antibodies raised are of limited heterogeneity and whether differences in inhibitability by different DNP-ligands can be detected. 3) to determine what factors control the acquisition by the gut-associated lymphoid tissues of mice of a preponderance of lgA producing plasma cells.